The present invention relates to computer systems and, more particularly, to interface apparatus for use in such a computer system.
In computer systems, there is provided one or more control units having authority over several subordinate units. The control unit may be the central processor or some lower level control unit. The subordinate units may, in fact, be any of a number of different types of peripheral devices. In operation, such a control unit communicates with the subordinate units over a communication bus. The control unit selects one of the subordinate units, addresses it and issues requests and command signals. It then awaits an acknowledge signal from the addressed subordinate unit to indicate that the command has been executed. In order to prevent the control unit from getting hung-up in the event of a malfunction or absence of the addressed subordinate unit from which no acknowledge signal is received, means must be provided for freeing the control unit. To that end, a timing unit has been incorporated into the control unit to effect a release of the control unit if no acknowledge signal has been received by a predetermined time after the issuance of the command signal. The subordinate units do not all have the same response time. For example, some of the units may respond in less than a microsecond while others may require several milliseconds. In the systems heretofore provided, to accommodate the wide range of response times, the timer was set to run out after a period slightly longer than the longest response time of any of the subordinate units associated therewith. That, however, effectively wastes a great amount of time slowing the entire system to the speed of the slowest element.